Physical oceanography studies have used the index of refraction as a means of determining the density of seawater for decades, although only recently have practical instruments with suitable accuracy been developed. In general terms, these instruments use one of three refractometry principles; critical reflection measurements at a single wavelength, speed of light measurements at a single wavelength, and critical reflection measurements using a broadband source.
In conventional refractometers, the variation of the critical angle is measured as a function of the external index of refraction, the measurement being performed using a monochromatic source. This method, generally accurate to the fifth decimal place, is used in commercial laboratory instruments as well as in industrial process control. In the 1980's an in-situ device was made based on this principle which used a solid-state beam-position indicator and was accurate to the sixth decimal place; however, the mechanical nature of angular measurements make them subject to errors from oceanic pressure and temperature changes.
The index of refraction is defined as the ratio of the speed of light in vacuum to that in the medium in question. Unfortunately, this parameter cannot be easily measured. However, with the help of a reference beam with which to compare the speed of the sensing beam, the phase difference between the two beams can be determined and becomes a very sensitive measure of the index of refraction. In the early 1990's a modified interferometer was used to measure the index of refraction to the seventh decimal place in the laboratory and to the sixth decimal place in-situ. As these interferometric methods measure the index of refraction relative to a fixed value in the reference beam, pressure and temperature changes can affect the result and thus limit the overall in-situ accuracy to the 10−6 range.
The third refractometry principle spectrally decomposes a broadband ‘white’ sensing beam reflected at the nominal critical angle from a flat window and determines the wavelength at critical reflection. This method exploits the differing dispersions of the indices of the glass window and the water that is external to the window. The primary benefits of this method are in its simplicity and its ability to perform in-situ measurements with an accuracy in the 10−6 range.
Although all of the afore-described techniques provide means for measuring the index of refraction of seawater, none of them provide the desired level of accuracy for an in-situ oceanographic instrument. Accordingly, what is needed in the art is an in-situ oceanographic instrument that can simply and reliably measure the index of refraction of seawater to the desired level of accuracy. The present invention provides such an in-situ instrument.